Experimental searches for GUT monopoles

We review recent experiments sensitive to slow, massive magnetic monopoles such as the relic big-bang monopoles predicted by grand unified theories (GUTs). Superconducting flux detectors now set flux upper limits of 3.7 x 10-11 cm-2sr-1s-1 (90% C.L., here and hereafter), independently of velocity. Searches with scintillator and proportional counter arrays are velocity-dependent for reasons of electronic practicality and the decrease of ionizing energy loss with decreasing monopole velocity, but in general much larger arrays can be assembled. Limits down to (2 to 4) x 10-14 cm-2sr-1s-1 have been obtained for v∼10-3c, substantially higher than the ''Parker bound'' resulting from galactic field regeneration time constraints